Interesting, I was eyeing the reamrkable as well, but for drawing. It's hackable, it runs Linux right? So it might be possible to hack into a portable Zettelkasten with good UX! :)
Oh yes definitely, it comes with root-access by default (and Vim preinstalled!). And there are interesting open-source libraries available to hack in new features: https://github.com/canselcik/libremarkable